1. Field of the Invention
The present invention relates to a process for etching and particularly to a process for etching suitable for use in the etching of a gate film, a tungsten alloy film, a silicon substrate, etc.
2. Description of the Prior Art
A gate film has hitherto been etched, for example, by a process disclosed in Japanese Patent Laid-Open No. 154730/1987 wherein the etching of a polycrystal silicon layer, i.e., a gate film, is conducted through the use of an etching gas comprising a mixture of SF.sub.6 or NF.sub.3 as an etching gas with C.sub.2 Cl.sub.3 F.sub.3 (trade name: Flon 113) or Flon 114 (trade name) as a depositing gas.
On the other hand, a silicon substrate has been etched, for example, by a process disclosed in Japanese Patent Laid-Open No. 299343/1988 wherein the etching is conducted with a gas mixture of NF.sub.3 or SF.sub.6 as an etching gas with at least one of CHF.sub.3, CH.sub.2 F.sub.2, CH.sub.3 F, C.sub.2 H.sub.2 F.sub.2, C.sub.2 HF.sub.3, C.sub.2 H.sub.3 F, C.sub.2 H.sub.5 F and C.sub.2 H.sub.4 F.sub.2 as a depositing gas.
In the etching of a gate film in the above-described former prior art, gases subject to fluorocarbon gas regulations established in recent years, such as Flon 113 or Flon 114, are used, but these gases cannot be used in the future, so that it has become an urgent necessity to develop a process wherein use is made of a gas other than the above-described gas species subject to fluorocarbon gas regulations.
The latter prior art discloses the etching of a silicon substrate with a gas not subject to such regulations. In this process, although a damage to a silicon substrate in trench etching of silicon is taken into consideration, no consideration is given to an improvement in the etching performance and a reduction in the amount of deposits particularly in the fabrication of a gate film, so that no sufficient etching performance can be attained.
Specifically, as shown in FIGS. 10 (a) to (d), when a gate film is etched through the use of the conventional etching gas, gate formation proceeds as the etching proceeds. For example, a side wall film gradually deposits on the side wall of a WSi.sub.x film 4. The formed deposits 6 function as if they were a mask, and the width of the WSi.sub.x film 4 which is not removed is gradually increased. This causes the WSi.sub.x film 4 to be tapered as shown in FIG. 10 (e) when subjected to ashing to remove a mask 5, which makes it impossible to attain perpendicular etching. Further, deposition occurs also on the bottom to be etched, and this unfavorably inhibits an improvement in the etching rate.